This invention relates to a case hardened roller cutter for a rotary drill bit and method of making, and more particularly to case hardened surfaces of a tungsten carbide insert type roller cutter and method of making same from a high hardenability steel.
Heretofore, roller cutters for rotary drill bits have been primarily of forged low carbon steels, i.e., steels having a carbon content between 0.10 and 0.20 per cent carbon, and such low carbon steel have commonly been case hardened by carburizing, quenching, and tempering processes. As well known, carburizing increases the carbon content of the surface of a steel and provides case hardening. Case hardening obtains a hard, wear resistant surface with a tough interior and low carbon steels have been commonly used heretofore for roller cutters. However, stringent hardenability requirements for low carbon carburizing grade steels are necessary to achieve a suitable strength/toughness balance in the core properties of insert type roller cutters. Failure to achieve the optimum level disposes the cutter toward either insert loss or cutter cracking. A variety of carburizing steel grades have been used to fabricate insert-type roller cutters, including PS-55, AISI 9310, 9315, 4815, and 4820.
A suitable steel is selected so that after carburizing of the bearing surfaces, and upon quenching and tempering the roller cutter for obtaining a case hardness of around RC60, the core yield strength of the roller cutter will be at least around 150 KSI. The chemistry of the steel is critical in obtaining the desired yield strength while maintaining suitable toughness. After heat treating, insert holes or sockets are drilled in the exterior of the cutter body for insertion of tungsten carbide inserts. Finally, the bearing races are finish ground to specified size and finish. The rolling cutter bearing race formed as above has a case hardened surface which is suited for use as a race for rolling bearings but which is often not sufficient when used as a sliding or friction bearing surface, particularly as the main friction bearing surface of a rock bit, without adding some feature to reduce the friction. If such a case hardened main friction bearing surface on the roller cutter is run directly against a typical journal which is formed of case hardened steel or steel with a welded on layer of stellite, experience has shown that the frictional heat generated is too high for such a bearing surface to function satisfactorily in a roller cutter drill bit under conditions normally encountered during operation.
Different methods or materials have been utilized heretofore for such main friction bearing surfaces of roller cutters for reducing the friction in such bearing surfaces to acceptable levels. Such methods have included, for example, the addition of floating bushings having an anti-galling metal thereon, the boronizing of the outer bearing surface after the carburizing, and the use of an alternating inlay of anti-galling metal with the carburized case hardened bore of the roller cutter.
Also, bearing surfaces for roller cutters or cones have been case hardened heretofore by high energy sources. For example, in U.S. Pat. No. 4,303,137 a method is disclosed for making a tungsten carbide insert rolling cutter using medium to high carbon steels for improved hardenability and hardness control. The method described in U.S. Pat. No. 4,303,137 selectively case hardens the ball race surface by using a high energy source such as induction heating, electron beam or laser beam to rapidly heat the surface for austenizing a surface layer which is rapidly cooled for case hardening the surface. Such a case hardened surface is similar in physical structure and chemistry to that obtained by carburizing a low carbon steel and is suitable for a rolling element bearing surface, but if used on the main friction bearing surface of the rolling cutter excessive friction is generated unless some other feature is added to reduce friction such as indicated above. Additionally, the method set forth in U.S. Pat. No. 4,303,137 for hardening the ball race of a medium to high carbon steel entails a number of fixturing and control problems which significantly affect process economics in a production environment.
Ion nitriding generally has been used heretofore for the surface hardening of metal for various applications such as for use in gears, valves, gun barrel interiors, and on crankshafts for internal combustion engines. However, bearing surface applications historically have been limited to hydrodynamic or hydrostatic design and/or relatively light loading as compared to the relatively high loading on the main friction bearing races of drill bits.